Plucked straight out of a...
Plucked straight out of a 12-second ’65 Mustang coupe, this 347 stroker perfectly represents the kind of motor found in your typical street machine. The Brodix LH17 heads were mighty impressive, good for 466 hp with a relatively tame cam. Based on how well they performed on our 347, it’s safe to presume that they’d easily support 550 hp in a 408 ci or larger combo with a more aggressive camshaft.
What started out as a simple cam test turned out to be a real head-scratcher, as the results were puzzling for a number of reasons. First off, the biggest cam didn’t make the most power. Secondly, the Thumpr’s specs suggest that it should run out of breath the earliest. Instead, it proved to be the most rev-happy cam of the bunch. The most obvious distinguishing factor between the Thumpr and its test mates is its tight LSA. Whereas Cam A and Cam B both shared 112-degree LSAs, the Thumpr was ground at 107 degrees. Tighter lobe-separation angles generally boost low-end torque at the expense of top end power. Furthermore, compared to Cam B, the Thumpr has an earlier intake valve closing point of 65 degrees after bottom dead center as opposed to 70 degrees ABDC. Intake valve closing is by far the most important of the four valve events in terms of power output, and a later IC generally improves top end power while sacrificing low-end torque. Again, neither of these scenarios held true during our test.
Seeking the expertise of someone much smarter than the typical journalist, we ran the results by SAM’s Head Instructor Judson Massingill for a more educated opinion. As it turns out, it’s all about cylinder pressure. “The intake duration of the Thumpr cam falls right in between Cam A and Cam B, so duration isn’t what’s accounting for the power difference. Plus, the Thumpr has the least amount of lift, so you can throw that out of the equation as well,” he says. “What’s going on in this motor is that the tighter LSA and increased overlap of the Thumpr is helping pull more air through the intake valve. The improvements in intake charge filling and cylinder pressure is what’s bumping up both torque and horsepower. Tight LSA cams usually drop off at high rpm, but the reason that’s not happening in this instance has to do with cylinder pressure. On a relatively low compression motor like this 347, you don’t have much cylinder pressure to begin with, so having a wide LSA really hurts you. With a tighter 107 LSA, the increase in cylinder pressure helps make up for the low compression ratio of the motor. If this 347 had a higher compression ratio, the tighter LSA of the Thumpr would have made less of a difference on the dyno.”
Formerly occupying the cam...
Formerly occupying the cam bores in the 347 was a mild hydraulic flat-tappet grind that fell on its face above 5,500 rpm. The three COMP hydraulic roller cams promised superior top end pull in addition to improved low-end grunt, thanks to their faster ramp acceleration rates.
Judson says, what makes this best-of-both-worlds scenario possible, in which low-end torque doesn’t come at the expense of top end power, are cylinder heads that flat-out get the job done. “When you have heads that move a lot of air, you can get by with a short-duration camshaft because tightening up the LSA increases cylinder pressure everywhere. You always want the best heads possible with the least amount of cam as possible,” he says. “In an application like this 347, the only reason to run a wider LSA is to smooth out the idle quality. As long as you can get it to hook, the Thumpr would be at least two-tenths quicker at the track than the other cams in this test. Not only does it produce more midrange power, it also carries that power to a higher rpm.”
Ultimately, no amount of speculating is an adequate substitute for good old-fashioned dyno testing. Since speculation doesn’t always hold up on the dyno, it’s not a bad idea to dyno test multiple cams in your engine combo of choice before deciding on the perfect grind for your application. Compared to what it costs to build a motor, cams are relatively inexpensive, so it’s cheap experimentation.
Although hot rodders are penny...
Although hot rodders are penny pinchers by nature, from a tuning perspective, the $500 or so it costs to dyno test an engine is often well worth the investment. A huge perk of engine dyno testing is that it allows quick and easy timing changes. Cam A and Cam B performed best with 2 degrees of advance, while the Thumpr preferred 1 degree of advance.
Before pulling the handle,...
Before pulling the handle, SAM students set the ignition timing to 34 degrees of advance. With a conservative compression ratio of 9.6:1, the 347 ran hard all day long on 93-octane pump gas.
Fueling the 347 was a 4150-series...
Fueling the 347 was a 4150-series Holley 750-cfm carb. Pulling the spark plugs after the first round of pulls revealed that the air/fuel ratio was so spot-on that it wasn’t even necessary to re-jet the carb.
While they probably hurt peak...
While they probably hurt peak power numbers a bit, we dyno tested the 347 with the same 1⅝-inch headers that will be used in the car. The Brodix heads have two different sets of bolt patterns to fit a variety of headers.
The first series of dyno pulls...
The first series of dyno pulls with the 224/232-at-.050 cam (the small Cam “A”) sent the 347 into valve float at 5,900 rpm. SAM students busted out the valvespring pressure tester to discover just 110 pounds of seat pressure, and 240 pounds over the nose. To correct the situation, students shimmed the springs .120 inch, which effectively increased seat and open pressure to 150 and 340 pounds, respectively. This eliminated all vestiges of float for the rest of the test session.
Granted, they didn’t contribute...
Granted, they didn’t contribute to the horsepower tally one bit, but COMP’s slick cast-aluminum valve covers (PN 282) added some badly needed eye candy to a rather pedestrian-looking motor. They provided plenty of clearance for the rockers and springs, and include a set of integrated breathers. At $160 a set, they’re hard to pass up.
Cam A and Cam B both produced peak horsepower in the 6,100- to 6,200-rpm range, but the smaller grind falls off a table after peak while the larger cam does not. By 6,300 rpm, Cam A is down to 415 hp, while Cam B is still making 460 hp. That 45 hp difference is far greater than the 27hp gap posted by Cam B at peak power, proving once again that peak power is just part of the equation. The Thumpr (Cam C), on the other hand, clearly destroys the other two cams everywhere in the power and torque curve.